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Abstract

The effective use of agricultural residues is desirable. In this study, the manufacturing of mouldings using sweet sorghum bagasse powder and citric acid as an adhesive was attempted. The effects of the application method and citric acid content on the bending properties and water resistance of the mouldings were investigated. Powder and liquid applications were used. The citric acid solid content based on the total weight of the mouldings was adjusted to 0, 15, 20, 25, and 30 weight percent. The dumbbell-shaped moulding was manufactured at 200°C and 4 MPa for 10 minutes. The relationship between the bending properties and citric acid content differed depending on the application method; the powder application was effective for obtaining high bending properties with a lower citric acid content. The water resistance of the moulding was greatly affected by the application method when a low citric acid content was used, and it was affected by the citric acid content when a high citric acid content was used. According to the data measured from Fourier-transform infrared spectroscopy, it was easy to contain unreacted citric acid in the moulding with the powder application, and an ester linkage formed easily when the liquid application was used. It was suggested that the citric acid tends to work as an adhesive in the powder application, and it tends to modify the bagasse powder in the liquid application.

Sorghum (Sorghum spp.) has been used as food and animal feed, and it is expected to be a promising bioresource. However, a large amount of bagasse has been generated as an agricultural residue of the use of sorghum. Recently, use of the bagasse in biopellets and animal feed was attempted (Almodares and Hadi 2009, Theerarattananoon et al. 2011, Houx et al. 2013). However, these uses were not very effective in terms of the carbon reserve due to the short-term nature of the consumption of these products. The use of bagasse as a raw material for wood-based materials has also been attempted. Kusumah (Kusumah et al. 2016, 2017a, 2017b) has researched the development of particleboard using bagasse. However, a residue powder is generated when making the particles used as a raw material of particleboard. The bagasse powder should also be investigated as a raw material for wood-based materials.

Generally, synthetic resin adhesives such as formaldehyde-based resins have been used for wood-based materials because of their high level of adhesiveness. However, most raw materials are derived from fossil resources. Considering the various adverse effects of fossil resources, it is desirable to use biobased adhesives. The development of various biobased adhesives using bioresources such as lignin, tannin, protein, and polysaccharides has been researched (Pizzi 2006, 2016; Zhong and Sun 2007; Kalami et al. 2017). However, conventional biobased adhesives often require chemical substances derived from fossil resources in order to achieve good adhesiveness (Berard et al. 2011). Furthermore, these chemical substances may cause environmental pollution and health problems (Shebe 2013). Therefore, the development of a new biobased adhesive made without using chemical substances derived from fossil resources should be investigated.

Recently, it was reported that citric acid can be used as a wood adhesive (Umemura et al. 2012a, 2012b; Umemura and Kawai 2015; Del Menezzi et al. 2018). Citric acid is derived from bioresources and has been known to be a relatively safe substance. Citric acid also has a relatively cheap price, so it will not burden the cost of raw materials. Particleboards and mouldings were manufactured using citric acid, and the good mechanical and physical properties of these materials were clarified (Umemura et al. 2012a, 2012b, 2012c; Umemura and Kawai 2015; Kusumah et al. 2016, 2017a, 2017b; Widyorini et al. 2016, 2018). According to previous reports, citric acid powder was used when manufacturing wood and bark mouldings (Umemura et al. 2012a, 2012c). Mouldings with citric acid as adhesive are expected to be used as a substitute for conventional wood plastic composite product. Liquid adhesive application is a common method when manufacturing wood-based materials, and the bending properties and water resistance of mouldings made using powder and liquid adhesive applications have never been compared. The main objective of this paper is to develop moulding using sweet sorghum bagasse as a raw material and citric acid as an adhesive. The citric acid content was varied to find a suitable formula for the moulding and also clarified the effects of different application methods. In this study, an attempt was made to manufacture moulding composed of sweet sorghum bagasse powder and citric acid. The effects of the application method and citric acid content on the bending properties and water resistance of the moulding were investigated.

Materials and Methods

Materials

Sweet sorghum (S. bicolor [L. Monech]) bagasse from squeezed sorghum stalks was obtained from a research field at the Innovation Center of the Indonesian Institute of Sciences in Bogor, Indonesia. The sweet sorghum bagasse was pulverized, and powder was obtained using a 250-μm sieve (60 mesh). Citric acid (anhydrous, special grade reagent) was purchased from Nacalai Tesque, Inc. (Kyoto, Japan) and was also pulverized to <250 μm using a sieve. Both powders were vacuum-dried at 60°C for 15 hours. The moisture content of the sweet sorghum bagasse powder was 4 percent.

Moulding preparation

Two kinds of mouldings were prepared under the following conditions. Type A: The sorghum bagasse powder was mixed with citric acid powder using a mill. The mixture powder was poured into a dumbbell-shaped mould (A type) of the Japanese Industrial Standard K 7139 (Japanese Standards Association 2009), and the mould was hot pressed. Type B: A citric acid water solution with a concentration of 5 weight percent (wt%) was mixed with sorghum bagasse powder, and the mixture was dried in an oven at 80°C for 12 hours. The agglomerated mixture was pulverized using a mill. The mixture powder was poured into a dumbbell-shaped mould (A type), and the mould was hot-pressed. The citric acid solid content based on the total weight of the mouldings was adjusted to 0, 15, 20, 25, and 30 wt%. The details of the formulations are shown in Table 1. According to a previous report (Umemura et al. 2012a), the pressing condition was set to 200°C and 4 MPa for 10 minutes. The mouldings obtained were conditioned in a desiccator with silica gel. Three mouldings were manufactured for each variation.

Table 1 Formulation of the mouldings.
Table 1

Bending test

Both edges of the dumbbell-shaped moulding were cut, and rectangular specimens of 80 by 10 mm were prepared. A static three-point bending test was carried out with a span of 50 mm and a loading speed of 5 mm/min using an Instron 4411 universal testing machine (Instron, USA) with a 5-kN capacity load cell. The test was performed in triplicate, and the average values (with the standard deviations) of the modulus of rupture (MOR) and the modulus of elasticity (MOE) were calculated.

Repeated boiling treatment

Thickness and weight changes caused by the repeated boiling treatment were observed using the edges (about 20 by 20 mm) of the dumbbell-shaped moulding. The edges were vacuum-dried at 60°C for 15 hours prior to the treatment. The treatment conditions were boiling water immersion for 4 hours, drying at 60°C for 20 hours in an oven, boiling water immersion for 4 hours, and vacuum-drying at 60°C for 15 hours. The thickness and weight of the specimen were measured at each treatment. The test was performed in triplicate, and the average thickness and weight changes (with standard deviations) were calculated.

FT-IR spectroscopy

The mouldings were pulverized into powders before and after the repeated boiling treatment, and the powders were dried in a vacuum dryer at 60°C for 12 hours. Infrared spectra data were obtained with a Fourier-transform infrared (FT-IR) spectrophotometer (FT/IR-4200; JASCO Products Company) using the KBr disk method and were recorded with an average of 32 scans at a resolution of 4 cm−1.

Results and Discussion

Bending properties

Moulding specimens were cut on both sides of the dumbbell-shape and are shown in Figure 1. The color of the moulding specimens got darker with increasing citric acid content, irrespective of whether they were Type A or B. This would be mainly due to the oxidation of the sorghum bagasse powder and the degradation of citric acid. Type A moulding (a) was darker than Type B moulding (b), especially with 25 and 30 percent citric acid contents. This seemed to be the influence of the citric acid powder in the moulding.

Figure 1.—. Moulding specimens for bending test: (a) Type A mouldings; (b) Type B mouldings.Figure 1.—. Moulding specimens for bending test: (a) Type A mouldings; (b) Type B mouldings.Figure 1.—. Moulding specimens for bending test: (a) Type A mouldings; (b) Type B mouldings.
Figure 1 Moulding specimens for bending test: (a) Type A mouldings; (b) Type B mouldings.

Citation: Forest Products Journal 70, 2; 10.13073/FPJ-D-19-00060

Table 2 shows the average density and standard deviation of the moulding specimens used for the bending test. The moulding density ranged from 717 to 1,134 kg m−3. It increased with increasing citric acid content until 20 wt%, and then remained at an almost constant value, irrespective of the preparation type.

Table 2 Density of the specimens for bending test.
Table 2

Figure 2 shows the relation between the citric acid content and the bending properties of Type A and B mouldings. The average MOR and MOE values of the moulding made only of sweet sorghum bagasse (0 wt%) were 3.30 MPa and 0.71 GPa, respectively. This indicated that the moulding had very low bending properties. The maximum MOR of Type A mouldings was obtained at a 20 wt% citric acid content. The average value and average specific MOR (considering density) were 41.95 and 37.14 MPa, respectively. For Type B mouldings, the maximum MOR was obtained at a 25 wt% citric acid content. The average value and average specific MOR were 39.06 and 34.51 MPa, respectively. The maximum MOE of Type A mouldings was obtained at a 20 wt% citric acid content. The average value and average specific MOE considering density were 7.61 and 6.73 GPa, respectively, whereas for Type B mouldings, the maximum MOE was obtained at 30 wt% citric acid content. The average value and average specific MOE were 7.20 and 6.33 GPa, respectively. Therefore, it was clarified that the development of bending properties as a function of citric acid content differed depending on application method. Based on the specific MOR and MOE results, high bending properties were obtained by the powder application even with a low citric acid content. The reason for this seems to be that citric acid powder acted as a suitable adhesive. Compared with the previous research on moulding made of Acacia mangium wood and citric acid powders (Umemura et al. 2012a), superior bending properties were recognized at a citric acid content of 20 wt%. On the other hand, a much greater amount of citric acid was needed to develop high bending properties in the liquid application. This would be due to the penetration of citric acid into the sorghum bagasse powder.

Figure 2.—. Relationship between the citric acid content and the bending properties of Type A and B mouldings. (a) Modulus of rupture (MOR). (b) Modulus of elasticity (MOE). (c) Specific MOR. (d) Specific MOE. Error bars indicate the standard deviation for each treatment.Figure 2.—. Relationship between the citric acid content and the bending properties of Type A and B mouldings. (a) Modulus of rupture (MOR). (b) Modulus of elasticity (MOE). (c) Specific MOR. (d) Specific MOE. Error bars indicate the standard deviation for each treatment.Figure 2.—. Relationship between the citric acid content and the bending properties of Type A and B mouldings. (a) Modulus of rupture (MOR). (b) Modulus of elasticity (MOE). (c) Specific MOR. (d) Specific MOE. Error bars indicate the standard deviation for each treatment.
Figure 2 Relationship between the citric acid content and the bending properties of Type A and B mouldings. (a) Modulus of rupture (MOR). (b) Modulus of elasticity (MOE). (c) Specific MOR. (d) Specific MOE. Error bars indicate the standard deviation for each treatment.

Citation: Forest Products Journal 70, 2; 10.13073/FPJ-D-19-00060

Water resistance

Figure 3 shows the thickness changes that occurred in Type A and B mouldings in a repeated boiling treatment. The sorghum-only moulding decomposed completely during the first boiling treatment, whereas for both types of moulding bonded with citric acid, the form was maintained until the treatment was finished. The change in the thickness of the mouldings was greatly suppressed with the addition of citric acid. This was the case in both types of mouldings. Type A moulding with a 15 wt% citric acid content had a large thickness change compared with that of Type B moulding. A small amount of citric acid powder was insufficient to suppress the thickness change of the sorghum bagasse powder; the citric acid solution might have modified the sorghum bagasse powder hydrophobically through easy penetration. With a citric acid content of 20 to 30 percent, Type A moulding (Fig. 3a) showed a thickness change from 5.26 to 15.59 percent in the first boiling treatment and from −6.81 to 0.62 percent after being finally dried. However, Type B mouldings (Fig. 3b) in those citric acid contents maintained a thickness change from 6.27 to 18.10 percent in the first boiling treatment and from −5.54 to 3.35 percent after being finally dried. As a whole, Type A mouldings had a lower thickness change than Type B mouldings. The reason for this seems to be that sorghum bagasse powders were strongly bonded by citric acid powder.

Figure 3.—. Thickness change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.Figure 3.—. Thickness change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.Figure 3.—. Thickness change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.
Figure 3 Thickness change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.

Citation: Forest Products Journal 70, 2; 10.13073/FPJ-D-19-00060

The weight changes of Type A and B mouldings are shown in Figure 4. The weight change in both types of mouldings was greatly suppressed by the addition of citric acid, especially in the boiling processes. In the case of the Type A moulding with a 15 wt% citric acid content (Fig. 4a), the weight change after the first boiling treatment was 58.7 percent and the value following the second treatment was 62.9 percent. The final dried weight change was −16.6 percent. The Type B moulding with a 15 wt% citric acid content (Fig. 4b) had better resistance, especially under boiling. The weight change after the first boiling treatment of the Type B with that citric acid content was 39.9 percent, and the value following the second treatment was 45.4 percent. The final dried weight change was −19.8 percent. The weight change was greatly suppressed in both types of mouldings with a 20 wt% citric acid content and above. Both types of mouldings had similar values of 11.0 to 23.6 percent in the first boiling treatment and −23.2 to 34.1 percent after being finally dried. This value grew smaller when the citric acid content was increased. The negative value was due to some elution from the moulding that occurred during the treatment. In a comparison between Types A and B, it was recognized that the final dried weight decreases of Type A were slightly larger than those of Type B. This means that the Type A moulding contained a large soluble component.

Figure 4.—. Weight change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.Figure 4.—. Weight change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.Figure 4.—. Weight change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.
Figure 4 Weight change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.

Citation: Forest Products Journal 70, 2; 10.13073/FPJ-D-19-00060

Bonding mechanism

The effect of the application method of the citric acid on the bonding mechanism was investigated by FT-IR. The result is shown in Figure 5. According to the previous articles, the peak at around 1,718 cm−1 was attributed to C = O stretching derived from the carboxyl group and/or the C = O ester group (Žagar and Grdadolnik 2003). The peak at approximately 1,206 cm−1 was assigned to the C-O stretch in esters and −OH bending motions (Schwanninger et al. 2004). In the spectra of the mouldings (5a), the peak intensities at 1,718 and 1,206 cm−1 in Type A were higher than those in Type B. Considering the same citric acid content, unreacted citric acid and/or ester linkages seemed to be contained significantly in the moulding of Type A. In the spectra of the mouldings after repeated boiling treatments (Fig. 5b), the peak intensities at 1,718 and 1,206 cm−1 in Type A were obviously smaller than those in Type B. Therefore, Type A contained a large amount of unreacted citric acid that would be leached during the treatment. This finding supports the large weight loss shown in Figure 4a. On the other hand, the peak intensities at 1,718 and 1,206 cm−1 in Type B were similar even though they were measured after repeated boiling treatments (Fig. 5b). This indicates that the peaks were mainly derived from ester linkages. Consequently, it was suggested that liquid application was effective for the formation of ester linkages.

Figure 5.—. Infrared spectra of the mouldings. (a) Mouldings with 20 wt% citric acid and 0 wt%. (b) Moulding with 20 wt% citric acid after repeated boiling treatment.Figure 5.—. Infrared spectra of the mouldings. (a) Mouldings with 20 wt% citric acid and 0 wt%. (b) Moulding with 20 wt% citric acid after repeated boiling treatment.Figure 5.—. Infrared spectra of the mouldings. (a) Mouldings with 20 wt% citric acid and 0 wt%. (b) Moulding with 20 wt% citric acid after repeated boiling treatment.
Figure 5 Infrared spectra of the mouldings. (a) Mouldings with 20 wt% citric acid and 0 wt%. (b) Moulding with 20 wt% citric acid after repeated boiling treatment.

Citation: Forest Products Journal 70, 2; 10.13073/FPJ-D-19-00060

Based on the results obtained, the effects of the application method and citric acid content on the bending properties and water resistance of the moulding can be explained as follows. In the powder application, citric acid powder of 15 wt% acts as an adhesive between the bagasse powders, resulting in the development of bending properties to some degree. However, the citric acid content is not sufficient to develop high water resistance. With a citric acid content of 20 wt% or more, the citric acid powder acts as an adhesive sufficiently to result in high bending properties and water resistance, even though some unreacted citric acid remains. However, the bending properties tend to decrease with high citric acid content as a result of the decrease of the weight ratio of the bagasse powders. In the liquid application, the citric acid penetrates easily into the bagasse powder when the moulding is being prepared. As a result, citric acid of 15 wt% is an insufficient adhesive content to develop good bending properties. However, the ester linkage associated with citric acid inside of the bagasse powder led to small thickness and weight changes in repeated boiling treatments. When the citric acid content increased, good bending properties and water resistance were developed. Consequently, it was suggested that the powder application is effective to produce bagasse moulding with good bending properties and water resistance and with a low citric acid content.

Conclusions

The effects of the application method and citric acid content on the bending properties and water resistance of sweet sorghum bagasse moulding were investigated. The relationship between the bending properties and citric acid content differed depending on the application method; the powder application was effective for obtaining high bending properties with a lower citric acid content. The water resistance of the moulding with a low citric acid content of 15 wt% was greatly affected by the application method. However, the water resistance was affected by the citric acid content rather than the application method in mouldings with high citric acid contents. Based on the results of FT-IR, the moulding manufactured using the powder application contained a large amount of unreacted citric acid compared with that made using the liquid application. The formation of ester linkage was remarkable in the moulding made with liquid application. Powder application was effective for obtaining bagasse moulding having good bending properties and water resistance with a low citric acid content. It was suggested that, with the powder application, the citric acid tends to work as an adhesive, and with the liquid application it tends to modify the bagasse powder.

Acknowledgments

The authors would like to thank the Science and Technology Research Partnership for Sustainable Development (SATREPS) for financial support, and the Indonesian Institute of Sciences (LIPI) for supplying raw materials.

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<italic>Figure 1</italic>
Figure 1

Moulding specimens for bending test: (a) Type A mouldings; (b) Type B mouldings.

<italic>Figure 2</italic>
Figure 2

Relationship between the citric acid content and the bending properties of Type A and B mouldings. (a) Modulus of rupture (MOR). (b) Modulus of elasticity (MOE). (c) Specific MOR. (d) Specific MOE. Error bars indicate the standard deviation for each treatment.

<italic>Figure 3</italic>
Figure 3

Thickness change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.

<italic>Figure 4</italic>
Figure 4

Weight change during repeated boiling treatment. (a) Type A mouldings; (b) Type B mouldings.

<italic>Figure 5</italic>
Figure 5

Infrared spectra of the mouldings. (a) Mouldings with 20 wt% citric acid and 0 wt%. (b) Moulding with 20 wt% citric acid after repeated boiling treatment.

Contributor Notes

The authors are, respectively, Assistant Researcher (eko.widodo.100@gmail.com), Senior Researcher (sukma.surya@biomaterial.lipi.go.id), and Professor (subyakto@biomaterial.lipi.go.id), Research Center for Biomaterials, Indonesian Inst. Sci., Bogor; and Associate Professor, Lab. Sustainable Materials, Research Inst. Sustainable Humanosphere, Kyoto Univ., Japan (umemura@rish.kyoto-u.ac.jp [corresponding author]). This paper was received for publication in November 2019. Article no. 19-00060.